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                <h2 id="前置知识"><a href="#前置知识" class="headerlink" title="前置知识"></a>前置知识</h2><p>说到进程通信，我们首先得知道怎么是线程通信吧，那什么是线程通信呢？</p>
<p>顾名思义，进程通信（ InterProcess Communication，IPC）就是指进程之间的信息交换。</p>
<p>我们知道进程是分配系统资源的单位，包括内存地址空间（什么！你竟然不知道，等我😁😁😁），因此各进程拥有的内存地址空间相互独立。为了保证安全，一个进程不能直接访问另一个进程的地址空间。但是进程之间的信息交换又是必须实现的。</p>
<p>我们发现操作系统的内核空间是每个进程都共享的，所以进程之间要通信必须通过内核。下面我们来看看基于Linux 内核提供的常见的进程通信机制：</p>
<ul>
<li>管道（也称作共享文件）</li>
<li>消息队列（也称作消息传递）</li>
<li>共享内存（也称作共享存储）</li>
<li>信号量</li>
<li>信号</li>
<li>套接字（Socket）</li>
</ul>
<p>下面我们就来逐个击破。</p>
<h2 id="管道"><a href="#管道" class="headerlink" title="管道"></a>管道</h2><p>学过 Linux 命令的同学应该都知道，Linux 管道使用竖线 <code>|</code> 连接多个命令，这被称为管道符。如下面这样：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">command1 <span class="token operator">|</span> command2<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>以上这行代码就组成了一个管道，它的功能是将前一个命令（command1）的输出，作为后一个命令（command2）的输入，从这个功能描述中，我们可以看出管道中的数据只能单向流动，也就是半双工通信，如果想实现相互通信（全双工通信），我们需要创建两个管道才行。另外，通过管道符 <code>|</code> 创建的管道是<strong>匿名管道</strong>，用完了就会被自动销毁。并且，匿名管道只能在具有亲缘关系（父子进程）的进程间使用，也就是说，匿名管道只能用于父子进程之间的通信。</p>
<p>为了解决这个缺点，提出了<strong>有名管道</strong>，也称做 FIFO，因为数据是先进先出的传输方式。在使用命名管道前，先需要通过 mkfifo 命令来创建，并且指定管道名字：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">$ <span class="token function">mkfifo</span> myPipe<span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>myPipe 就是这个管道的名称，并且管道也是以文件的方式存在，使用 ls 命令可以查看到 myPipe 的文件类型是p，也就是管道（pipe）的意思。</p>
<p>接下来我们就可以往 myPipe 管道中写入数据了：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">$ <span class="token builtin class-name">echo</span> <span class="token string">"hello"</span> <span class="token operator">></span> myPipe  // 将数据写进管道
                         // 停住了 <span class="token punctuation">..</span>.<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>你会发现卡在了这里，这是因为管道里的内容没有被读取，只有当管道里的数据被读完后，命令才可以正常退出。于是开启另一个会话来读取这个管道里的数据：</p>
<pre class="line-numbers language-shell" data-language="shell"><code class="language-shell">$ <span class="token function">cat</span> <span class="token operator">&lt;</span> myPipe  // 读取管道里的数据
hello<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span></span></code></pre>

<p>可以看到，管道里的内容被读取出来了，并且 echo 那个命令也正常退出了。</p>
<p>从上面演示的来看，可以看出管道这种通信方式效率低，不适合进程间频繁地交换数据，但好处就是简单，很容易得知管道里的数据已经被另一个进程读取了。</p>
<p>下面我们从匿名管道的角度，来看看管道背后的原理：</p>
<p>匿名管道的创建，需要通过下面这个系统调用：</p>
<pre class="line-numbers language-c" data-language="c"><code class="language-c"><span class="token keyword">int</span> <span class="token function">pipe</span><span class="token punctuation">(</span><span class="token keyword">int</span> fd<span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>这里表示创建一个匿名管道，并返回了两个描述符，一个是管道的读取端描述符 <code>fd[0]</code>，另一个是管道的写入端描述符 <code>fd[1]</code>。注意，这个匿名管道是特殊的文件，只存在于内存，不存于文件系统中。如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113447443.png" alt="image-20220327113447443" style="zoom:80%;" />

<p>其实，所谓的管道，就是内核里面的一串缓存。从管道的一段写入的数据，实际上是缓存在内核中的，另一端读取，也就是从内核中读取这段数据。另外，管道传输的数据是无格式的流且大小受限。</p>
<p>看到这，你可能会有疑问了，这两个描述符都是在一个进程里面，并没有起到进程间通信的作用，怎么样才能使得管道是跨过两个进程的呢？</p>
<p>我们可以使用 fork 创建子进程，创建的子进程会复制父进程的文件描述符，这样就做到了两个进程各有两个 fd[0] 与 fd[1]，两个进程就可以通过各自的 fd 写入和读取同一个管道文件实现跨进程通信了，如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113503871.png" alt="image-20220327113503871" style="zoom:80%;" />

<p>上面这种模式只能一端写入，另一端读出，所以上面这种模式容易造成混乱，因为父进程和子进程都可以同时写入，也都可以读出。那么，为了避免这种情况，通常的做法是：</p>
<ul>
<li>父进程关闭读取的 fd[0]，只保留写入的 fd[1]；</li>
<li>子进程关闭写入的 fd[1]，只保留读取的 fd[0]；</li>
</ul>
<p>如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113517654.png" alt="image-20220327113517654" style="zoom:80%;" />

<p>所以说如果需要双向通信，则应该创建两个管道。但是我们上的  command1 | command2 中这两个命令并不存在父子关系啊，那这是怎么回事呢？</p>
<p>原因就是创建这两个命令的进程都是 shell 创建出来的子进程，父进程都是 shell，如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113530098.png" alt="image-20220327113530098" style="zoom:80%;" />

<p>对于匿名管道，它的通信范围是存在父子关系的进程。因为管道没有实体，也就是没有管道文件，只能通过 fork 来复制父进程 fd 文件描述符，来达到通信的目的。</p>
<p>对于命名管道，它可以在不相关的进程间也能相互通信。因为命令管道，提前创建了一个类型为管道的设备文件，在进程里只要使用这个设备文件，就可以相互通信。</p>
<p>不管是匿名管道还是命名管道，进程写入的数据都是缓存在内核中，另一个进程读取数据时候自然也是从内核中获取，同时通信数据都遵循先进先出原则。</p>
<h2 id="消息队列"><a href="#消息队列" class="headerlink" title="消息队列"></a>消息队列</h2><p>我们知道了管道的通信方式是效率低的，因此管道不适合进程间频繁地交换数据。对于这个问题，消息队列的通信模式就可以解决。比如，A 进程要给 B 进程发送消息，A 进程把数据放在对应的消息队列后就可以正常返回了，B 进程需要的时候再去读取数据就可以了。同理，B 进程要给 A 进程发送消息也是如此。如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113543628.png" alt="image-20220327113543628" style="zoom:80%;" />

<p>消息队列是保存在内核中的消息链表，在发送数据时，会分成一个一个独立的数据单元，也就是消息体（数据块），消息体是用户自定义的数据类型，消息的发送方和接收方要约定好消息体的数据类型，所以每个消息体都是固定大小的存储块，不像管道是无格式的字节流数据。如果进程从消息队列中读取了消息体，内核就会把这个消息体删除。</p>
<p>消息队列生命周期随内核，如果没有释放消息队列或者没有关闭操作系统，消息队列会一直存在，而前面提到的匿名管道的生命周期，是随进程的创建而建立，随进程的结束而销毁。</p>
<p>但是这种通信方式也有缺点：</p>
<ul>
<li>通信不及时</li>
<li>附件有大小限制</li>
</ul>
<p>消息队列不适合比较大数据的传输，因为在内核中每个消息体都有一个最大长度的限制，同时所有队列所包含的全部消息体的总长度也是有上限。</p>
<p>消息队列通信过程中，存在用户态与内核态之间的数据拷贝开销，因为进程写入数据到内核中的消息队列时，会发生从用户态拷贝数据到内核态的过程，同理另一进程读取内核中的消息数据时，会发生从内核态拷贝数据到用户态的过程。因此，如果数据量较大，使用消息队列就会造成频繁的系统调用，也就是需要消耗更多的时间以便内核介入。</p>
<h2 id="共享内存"><a href="#共享内存" class="headerlink" title="共享内存"></a>共享内存</h2><p>消息队列的读取和写入的过程，都会有发生用户态与内核态之间的消息拷贝过程。那共享内存的方式，就很好的解决了这一问题。</p>
<p>顾名思义，共享内存就是允许不相干的进程将同一段物理内存连接到它们各自的地址空间中，使得这些进程可以访问同一个物理内存，这个物理内存就称为共享内存。如果某个进程向共享内存写入数据，所做的改动将立即影响到可以访问同一段共享内存的任何其他进程。</p>
<p>我们来看看共享内存的原理：</p>
<p>首先，每个进程都有属于自己的进程控制块（PCB）和逻辑地址空间（Addr Space），并且都有一个与之对应的页表，负责将进程的逻辑地址（虚拟地址）与物理地址进行映射，通过内存管理单元（MMU）进行管理。两个不同进程的逻辑地址通过页表映射到物理空间的同一区域，它们所共同指向的这块区域就是共享内存。如图所示：</p>
<p><img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113628256.png" alt="image-20220327113628256" style="zoom:80%;" /><img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113637100.png" alt="image-20220327113637100"></p>
<p>不同于消息队列频繁的系统调用，对于共享内存机制来说，仅在建立共享内存区域时需要系统调用，一旦建立共享内存，所有的访问都可作为常规内存访问，无需借助内核。这样，数据就不需要在进程之间来回拷贝，所以这是最快的一种进程通信方式。</p>
<h2 id="信号量"><a href="#信号量" class="headerlink" title="信号量"></a>信号量</h2><p>用了共享内存通信方式，带来新的问题，那就是如果多个进程同时修改同一个共享内存，很有可能就冲突了。例如两个进程都同时写一个地址，那先写的那个进程会发现内容被别人覆盖了。</p>
<p>为了防止多进程竞争共享资源，而造成的数据错乱，所以需要保护机制，使得共享的资源，在任意时刻只能被一个进程访问。正好，信号量就实现了这一保护机制。</p>
<p>信号量其实是一个整型的计数器，主要用于实现进程间的互斥与同步，而不是用于缓存进程间通信的数据。</p>
<p>信号量表示资源的数量，控制信号量的方式有两种原子操作：</p>
<ul>
<li>一个是 P 操作，这个操作会把信号量减去 -1，相减后如果信号量 &lt; 0，则表明资源已被占用，进程需阻塞等待；相减后如果信号量 &gt;= 0，则表明还有资源可使用，进程可正常继续执行。</li>
<li>另一个是 V 操作，这个操作会把信号量加上 1，相加后如果信号量 &lt;= 0，则表明当前有阻塞中的进程，于是会将该进程唤醒运行；相加后如果信号量 &gt; 0，则表明当前没有阻塞中的进程；</li>
</ul>
<p>P 操作是用在进入共享资源之前，V 操作是用在离开共享资源之后，这两个操作是必须成对出现的。</p>
<p>我们首先来看看使用信号量实现互斥操作：</p>
<p>如果要使得两个进程互斥访问共享内存，我们可以初始化信号量为 <code>1</code>。</p>
<p><img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113637100.png" alt="image-20220327113637100"></p>
<p>过程如下：</p>
<ul>
<li>进程 A 在访问共享内存前，先执行了 P 操作，由于信号量的初始值为 1，故在进程 A 执行 P 操作后信号量变为 0，表示共享资源可用，于是进程 A 就可以访问共享内存。</li>
<li>若此时，进程 B 也想访问共享内存，执行了 P 操作，结果信号量变为了 -1，这就意味着临界资源已被占用，因此进程 B 被阻塞。</li>
<li>直到进程 A 访问完共享内存，才会执行 V 操作，使得信号量恢复为 0，接着就会唤醒阻塞中的线程 B，使得进程 B 可以访问共享内存，最后完成共享内存的访问后，执行 V 操作，使信号量恢复到初始值 1。</li>
</ul>
<p>再来看看信号量实现同步操作：</p>
<p>例如，进程 A 是负责生产数据，而进程 B 是负责读取数据，这两个进程是相互合作、相互依赖的，进程 A 必须先生产了数据，进程 B 才能读取到数据，所以执行是有前后顺序的。</p>
<p>那么这时候，就可以用信号量来实现多进程同步的方式，我们可以初始化信号量为 <code>0</code>。</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113650749.png" alt="image-20220327113650749" style="zoom:80%;" />

<p>具体过程：</p>
<ul>
<li>如果进程 B 比进程 A 先执行了，那么执行到 P 操作时，由于信号量初始值为 0，故信号量会变为 -1，表示进程 A 还没生产数据，于是进程 B 就阻塞等待；</li>
<li>接着，当进程 A 生产完数据后，执行了 V 操作，就会使得信号量变为 0，于是就会唤醒阻塞在 P 操作的进程B；</li>
<li>最后，进程 B 被唤醒后，意味着进程 A 已经生产了数据，于是进程 B 就可以正常读取数据了。</li>
</ul>
<h2 id="信号"><a href="#信号" class="headerlink" title="信号"></a>信号</h2><p>这里说的信号不同于上面的信号量，信号是进程通信机制中唯一的异步通信机制，它可以在任何时候发送信号给某个进程。通过发送指定信号来通知进程某个异步事件的发送，以迫使进程执行信号处理程序。信号处理完毕后，被中断进程将恢复执行。用户、内核和进程都能生成和发送信号。</p>
<p>信号事件的来源主要有硬件来源和软件来源。所谓硬件来源就是说我们可以通过键盘输入某些组合键给进程发送信号，比如常见的组合键 Ctrl+C 产生 SIGINT 信号，表示终止该进程；而软件来源就是通过 kill 系列的命令给进程发送信号，比如 kill -9 8080，表示给 PID 为 8080 的进程发送 SIGKILL 信号，让其立即结束。我们来查看一下 Linux 中有哪些信号：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113700728.png" alt="image-20220327113700728" style="zoom:80%;" />

<h2 id="Socket"><a href="#Socket" class="headerlink" title="Socket"></a>Socket</h2><p>前面提到的管道、消息队列、共享内存、信号量和信号都是在同一台主机上进行进程间通信，那要想跨网络与不同主机上的进程之间通信，就需要 Socket 通信了。</p>
<p>从计算机网络层面来说，Socket 套接字是网络通信的基石，是支持 TCP/IP 协议的网络通信的基本操作单元。它是网络通信过程中端点的抽象表示，包含进行网络通信必须的五种信息：连接使用的协议，源主机的 IP 地址，源进程的协议端口，目的主机的 IP 地址，目的进程的协议端口。</p>
<p>Socket 的本质其实是一个编程接口（API），是应用层与 TCP/IP 协议族通信的中间软件抽象层，它对 TCP/IP 进行了封装。它把复杂的 TCP/IP 协议族隐藏在 Socket 接口后面。对用户来说，只要通过一组简单的 API 就可以实现网络的连接。如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtianyu/img/img/image-20220327113759403.png" alt="image-20220327113759403" style="zoom:80%;" />

<p>好啦，到这里我们的进程间的6种通信方式就讲完啦，希望你能好好消化哦。🤪🤪🤪</p>
<blockquote>
<p>引用：</p>
<p><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzUxODAzNDg4NQ==&amp;mid=2247485318&amp;idx=1&amp;sn=0da0a684639106f548e9d4454fd49904&amp;chksm=f98e432ccef9ca3ab4e10734fd011c898785f18d842ec3b148c7a8ee500790377858e0dbd8d6&amp;scene=178&amp;cur_album_id=1408057986861416450#rd">https://mp.weixin.qq.com/s?__biz=MzUxODAzNDg4NQ==&amp;mid=2247485318&amp;idx=1&amp;sn=0da0a684639106f548e9d4454fd49904&amp;chksm=f98e432ccef9ca3ab4e10734fd011c898785f18d842ec3b148c7a8ee500790377858e0dbd8d6&amp;scene=178&amp;cur_album_id=1408057986861416450#rd</a></p>
<p><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzI0NDc3ODE5OQ==&amp;mid=2247485745&amp;idx=1&amp;sn=c87aa231b954f346cec7310ecf197b8a&amp;scene=19#wechat_redirect">https://mp.weixin.qq.com/s?__biz=MzI0NDc3ODE5OQ==&amp;mid=2247485745&amp;idx=1&amp;sn=c87aa231b954f346cec7310ecf197b8a&amp;scene=19#wechat_redirect</a></p>
</blockquote>

                
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